The sons and daughters of people who live very long lives tend to get the symptoms of Alzheimer's disease later than others, but they're not immune from the memory-robbing disease, according to a new study.

Based on comparisons of people in their 90s, their spouses, siblings, children and their children's spouses, researchers found that the offspring of people with exceptional longevity were about 40 percent less likely than peers to be cognitively impaired between ages 65 and 79.

"It's not necessarily that these individuals never become cognitively impaired, but what it seems like is that there is a delayed onset of cognitive impairment," said Stephanie Cosentino, of the Columbia University Medical Center in New York.

By the time the older generation of study volunteers were in their 90s, however, their risk of being cognitively impaired was fairly high.

So Cosentino's team projects that the kids of these long-lived individuals will have the same risk level as their parents if they enjoy similar longevity - that is, they'll no longer be protected.

Loosely defined, longevity means living beyond the average age of death among peers. In the U.S. today, for instance, a 65 year old man can expect to live to age 83, on average, and a woman to age 85.

As life expectancies continue to rise, few have investigated whether that means people live to those old ages cognitively "intact," Cosentino and her colleagues write in JAMA Neurology.

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Alzheimer's disease is diagnosed in about 5,000 Americans each year. It's the most common form of dementia, affecting more than 5 million Americans, according to the National Institute on Aging.

Both longevity and dementia risk have some degree of heritability.

For the new study, the researchers used data on cognitive impairment from 1,870 people who are part of the Long Life Family Study, which includes volunteer participants in New York, Massachusetts, Pennsylvania and Denmark.

In the U.S., subjects were recruited along with their (younger) siblings, their children and their siblings' children - as well as all available spouses in both generations, who serve as an unrelated comparison group without necessarily having a family history of longevity.

The study included 1,510 people with a family history of longevity and 360 of their spouses, but for this study, researchers used information on just the volunteers who were 89 years old or older when they were recruited.

A series of tests measured the participants' memory, recall abilities, language and processing skills.

Overall, the researchers found that about 6 percent of the volunteers' children were cognitively impaired between ages 65 and 79 years old, compared to 13 percent of their spouses and about 11 percent of their cousins.

Among the study's long-lived older generation, participants were just as likely to be cognitively impaired by about age 90 as their siblings or spouses.

"These families seem relatively protected, but once they reach extreme old age - say after 90 (years old) - their rates of cognitive impairment become comparable," Cosentino said.

The study cannot say whether a parent's longevity protected the offspring from becoming cognitively impaired in their 60s and 70s, but Cosentino said she and her colleagues want to know what makes them different from the others.

"Our current interest now is understanding the genetic basis of that delay," she said, adding that the hope is they could eventually help others delay memory problems as well.

Mary Sano, director of the Alzheimer's Disease Research Center at the Icahn School of Medicine at Mount Sinai in New York, told Reuters Health that the new study supports previous research on aging and dementia.

"This study and other studies have really shown we are moving in the direction of understanding the genetics and biology of successful aging," said Sano, who was not involved with the new study.

"I think the reason it's so promising is because we've put a lot of effort into studying diseases to find the silver bullet to prevent disease and very little work goes into people who are less likely to have the disease or less likely to develop the disease," she said.

"I think looking at this side is critical, because we haven't had much luck looking the other way," Sano said.